1. Field of the Invention
This invention pertains to packers employed in sealing off a drill string so that control of the well can be reestablished after the development of a downhole high pressure condition and more specifically to the packers employed with a drop-in check valve installed in a drill string.
2. Description of the Prior Art
Circulating fluids in an oil or gas well being drilled often provide an indication of whether there is a gas build-up that may result in the creation of a hazardous condition unless precautionary steps are taken. For example, when more fluid is returned to the surface over a given time than is being used in circulation, this can be an indication that there is developing a gas build-up downhole. When this occurs, it is desirable to change the circulating fluid to a heavier composition, which heavier fluid will hold back the gas pressure formation and therefore, "reestablish" or gain control of the well.
It is not possible to make the transition from regular fluid to heavy fluid instantaneously. Unless the fluid is changed quickly, however, there is no assurance that the heavy fluid will be in use soon enough to regain control of the well. Thus, it is desirable to shut off circulation of the fluid and block off the downhole well pressure while the well is reestablished. The device that is employed downhole in the drill string for this purpose is referred to as a drop-in check valve. The deeper the well, the better and more secure this drop-in check valve has to be.
To permit the use of a drop-in check valve, a landing sub is used in the place of or in combination with a regular joint of drill pipe at a location not too far from the bottom of a hole. Such a landing sub has a central bore or opening with approximately the same inside diameter as the inside diameter of the bores of other pipe joints in the drill string. However, the inside diameter is configured with an appropriate landing shoulder to capture a freely falling drop-in check valve subassembly. Such a subassembly is used to block off the well when the conditions are as described above or at a propitious time, i.e., hopefully, during pressure build-up, but before any blowout.
A telescoping slip on the top of the subassembly with laterally outwardly biased fingers is located around the tubular central body of the subassembly. These fingers release or lock within a locking recess beneath an inside, downwardly facing shoulder in the landing sub bore so that a pressure from downhole cannot move the sub upward beyond a predetermined position. The drop-in check valve is forced down in place against the downhole pressure using a pressure hose connected to the kelly, even though the assembly itself is not held in any manner and is free to fall.
The lower end of the drop-in check valve includes a ball valve assembly having three main parts, namely, an upper housing with a valve seat in its lower end, an upwardly biased ball for seating in the valve seat when there is a high pressure downhole condition, and a ball cage for holding the bias spring and ball. When the ball is pushed upwardly, the ball housing moves up and pushes the main body of the drop-in valve assembly up to seat a sloping body surface on substantially identically slanted inside surfaces of the slip fingers. It will be recalled that the slip fingers are locked in place against upward movement.
The lower end of the body surrounds the housing of the ball valve assembly, its cross-section depending into the annulus between the housing and the bore of the landing sub. Immediately below the body in the annulus is a packer or packer assembly and immediately below the packer assembly is the top end of the ball cage, which surrounds the lower end of the ball valve housing. Thus, upward pressure on the ball cage produces pressure on the packer or packer assembly. Upward axial pressure on the packer causes it to expand radially or laterally to the inside against the housing and to the outside against the landing sub, effectively blocking off the annulus.
A packer assembly in a drop-in check valve in the prior art included a soft elastomer ring contiguous with and below a hard elastomer ring. The top of the hard elastomer ring of this packer assembly has a truncated peak in the middle and includes sloping surfaces on either side of the central peak for resting on ring parts in the bottom end of the main body, at least one of which also moved slightly outwardly to contact the inside of the bore of the landing sub when placed under pressure by the expanding hard elastomer ring. Thus, the elastomer rings are blocked against longitudinal movement and pressed laterally outwardly to retain the pressure in the annulus between the check valve housing and the landing sub. Thus, with the ball of the check valve blocking the central bore, the entire drill string is blocked off.
Although the packer assembly just described proves satisfactory in environments where the pressure is less than about 10,000 psi and the temperature is less the 250.degree. F., pressures or pressures and temperatures above these levels can cause failure of the elastomers. Elastomers become soft under high pressure and temperature conditions and tend to extrude. When this occurs, it occurs with respect to the gap opposite the peak of the hard elastomer ring. The gap along the bore wall next to the movable piece above the hard elastomer ring and the gap below the soft elastomer ring along the bore wall and the top of the ball cage also allow possible extrusion of the respective rings to occur.
Therefore, it is a feature of the present invention to provide an improved packer, and especially an improved packer assembly in a drop-in check valve assembly, that includes a metal ring above and/or below the elastomer rings of the packer to minimize gap possibilities for the extrusion of the elastomers.
It is another feature of the present invention to provide an improved packer as noted above having expansible, overlapping metallic components that cap the top elastomer ring and forms a resulting metal-to-metal seal to prevent elastomer extrusion even under pressures up to 15,000 psi and temperatures up to 400.degree. F.
It is still another feature of the present invention to provide an improved packer as noted above including a bottom metallic ring for limiting the amount of extrusion flowing toward the downhole high pressure in the annulus between the lower part of the drop-in valve assembly and the bore of a landing sub affixed in the drill string.
It is yet another feature of the present invention to provide an improved packer as noted above having expansible, overlapping metallic components that cap the lower elastomer ring of the packer assembly in a similar manner to the capping of the upper elastomer ring.